For example, due to a drop in cost of lithium ion batteries having high energy density and situations in which a back-up time has to be secured by an emergency storage battery system at the time of wide-scale disasters, there has been increasing social demands for addition or replacement of a storage battery system using a lithium ion battery in an existing emergency storage battery system based on a lead storage battery. From such a background, there has been proposed a technique of using a lithium ion battery in an emergency storage battery system have been proposed.
However, in the conventional technology, when charging is performed in an emergency battery system in which storage battery devices each of which includes a lithium ion battery are connected in parallel, the lithium ion battery is exclusively charged for each storage battery device by a single charging device installed outside the storage battery device. For this reason, in the emergency storage battery system in which a large number of storage battery devices are connected in parallel, there is a problem in that a time necessary for charging is taken long. Further, when there is a potential difference between the storage battery devices, an electric current flows from a storage battery device of a charging target to another storage battery device, and so the other storage battery device is charged. Thus, there is a problem in that it takes a long time to charge the storage battery device of the charging target.